Serializable meta-annotation

[rsinukov]

https://youtrack.jetbrains.com/issue/KTOR-2704

[pdvrieze]

Overall a good idea. There is one challenge, if you use a custom serializer it doesn't make sense to provide any other values for the annotation (custom serializers don't have access to generated descriptors - it is broken in many ways, but if they are generic even more). As an alternative, why not use:

interface SerializerFactory<A: Annotation> {
   fun getSerializer<T>(type:KClass, annotation: A): KSerializer<T>
}

This provides the factory the opportunity to return the serializer needed for that specific type (and meta annotation).

[sandwwraith]

Obsolete test?

[sandwwraith]

Since JetBrains/kotlin#4583 goes to 1.7.0, I'll merge this when the corresponding release would be prepared.

[sandwwraith]

Transferred commit to #1979